Photoconductive camera tubes and methods of manufacture

ABSTRACT

The present invention is directed to a very thin photoconductive camera tube target formed of extrinsic semiconductor material. The resulting target has an enhanced contrast and signal-to-noise ratio.

Ilite States Patent 1 1 1 3,729,645

edington 1 Apr. 24, 1973 PHOTOCONDUCTIVE CAMERA TUBES 2,908,835 /1959 Weimer ..313/ AND METHODS OF MANUFACTURE 3,121,808' 2/1964 Kahng et al..... ..148/175 UX 131 4 l Inventor: Rowland W. Redington, Schenec- 964 Shomert 148/175 X FOREIGN PATENTS OR APPLICATIONS 1 1 Assigneer' General Electric p y, Sche- 1,300,566 6/1962 France ..313/65 nectady, NY.

[22] Filed; 1 19 4 Primary ExaminerCar1 D. Quarforth Assistant Examiner-l M. Potenza [21] Appl' AttorneyRichard R. Brainard, John F. Ahern, John P. Dellitt, Frank L. Neuhauser and Oscar B. Waddell [52] US. Cl ..315/11, 313/65 A [51] Int. Cl ..H0lj 31/48 [57] ABSTRACT [58] Field ofSearch ..317/235; 313/65,

The present invention is directed to a very thin 313/94 65 307/885 148/175 315/10 1 1 photoconductive'camera tube target formed of extrin- [56] References Cited sic semiconductor material. The resi lting target has 1 an enhanced contrast and signal-to-nolse ratio.

UNITED STATES PATENTS 8 Claims, 8 Drawing Figures 2,843,773 7/1958 Wardley ..313/65 reca l sing o/e ao/ Maze Grail Maze Cai/ leer/an Coils: l6

Refrige/alian Means Patented April 24, 1973 3,729,645

2 Sheets-Sheet l Deflection Coils Refrigeration Focusing Sole/ml Maze Coi/ Maze Coil k 3; g Row/0nd W Red/hymn His Aria/Hey Patented April 24, 1973 3,729,645

2 Sheets-Sheet 2 Support Grown Layer 7 l A A r \r Conduct/0n Band Ferm/Leve/ Yakehce Band //7 ve nfor R0 w/and W Red/n 9/00,

H/s A Home y.

PHOTOCONDUCTIVE CAMERA TUBES AND v METHODSOIFMANUFACTURE This invention relates to photoconductive camera tubes employing semiconductor targets and particularly to such tubes and targets which are sensitive to infrared radiation.

Certain semiconductor materials exhibit photoconductive properties, and therefore may be suitably employed for targets in television-type camera tubes. In a vi-dicontype tube, for example, radiation is received on one side of a semiconductor target through a relatively positive transparent electrode. The reception of illumination creates free charge carriers in the semiconductor at the points illuminated. Either an electron or when the illuminated picture element is scanned by the tubes electron beam, just enough electrons are deposited from the scanning beam to replace the nega tive charge removed in the preceding frame period by photoconduction. The instantaneous charge build-up,

capacitively coupled to the transparent electrode, constitutes the picture signal output in the usual vidicon.

Semiconductor materials of both the intrinsic or extrinsic type have been utilized as a photoconductive material in a camera tube target, a main consideration being the illumination wavelength which is to be detected. The extrinsic materials are so named because certain impurities are added to the semiconductor. Semiconductors of this type are more useful for detecting illumination or radiation at longer wavelengths, for example, in the infrared region.

The use of extrinsic semiconductor targets is convenient for detection in the infrared region because impurities can be added which create energy levels in the semiconductor which lie in close proximity to energy bands associated with the semiconductor. Thus infrared radiation may excite an electron from the semiconductor stable energy state or valence band to a shallow acceptor level provided by added impurity material, the acceptor energy level corresponding in its energy distance above the valence band to the energy of radiation in the infrared region. A conducting hole is thereby freed in the semiconductor valence band and acts as the current carrier for the photoconductive target.

Present semiconductor targets are generally to mils in axial dimension, and although these targets are thin enough to be useful in the detection of infrared radiation, there are several incentives for employing thinner camera tube targets. First, the maximum image resolution is inversely proportional to target thickness. Second, the peak signal-to-noise ratio, and thus contrast sensitivity, improves with the square root of target capacitance. Because of the fragility of present targets as made by presently known methods, including the problems of mounting them, these targets cannot be made much thinner.

Accordingly it is an object of the present invention to provide a very thin photoconductive camera tube target formed of extrinsic semiconductor material, and therefore a target having enhanced contrast and signalto-noise ratio.

In accordance with the present invention, a target is formed by epitaxially growing a thin semiconductor layer on a semiconductor base. The epitaxially grown layer, which can be as thin as half a mil, forms the only active portion of the semiconductor target. That is, only this layer provides charge carriers in response to a particular radiation. The semiconductor base forms a substantially transparent window through which illumination may pass before reception in the epitaxial layer.

In accordance with an aspect of the present invention, an initially n-type semiconductor layer is epitaxially deposited upon a p-type or weakly n-type base. Then an impurity for rendering semiconductor material p-type is added to the target for compensating the ntype impurity of the epitaxial layer, and rendering it effectively p-type. In accordance with a preferred embodiment of the invention, the p-type impurity added is copper, having first and second acceptor levels, one of which compensates for the donors of the initial n-type impurity. A second copper acceptor level is effective for rendering the camera tube target responsive to infrared radiation.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and'method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompany drawings wherein like reference characters refer to like elements and in which:

FIG. 1 is an elevational view, principally in cross-section, of an infrared camera tube and target constructed in accordance with the present invention,

FIG. 2 illustrates a process in forming the semiconductor target,

FIG. 3 is an elevational view partially broken away of such target,

FIG. 4 is an elevational view, principally in cross-section, of the semiconductor target coated with metal, specifically copper,

FIG. 5 is an elevational view, partially broken away, of the target with the metal coating removed and a connection made thereto,

FIG. 6 is an isometric drawing of the semiconductor target placed in its retaining member,

FIG. 7 is an energy diagram of the semiconductor target before the final doping thereof, and

FIG. 8 is an energy diagram for the semiconductor target after final doping.

Referring to FIG. 1, showing by way of example an infrared sensitive camera tube embodying features of the present invention, a long cylindrical glass envelope 1 is closed at one end by means of a tube base 2 providing connections 3 for electron gun structure 4 and electron multiplier output device 5. The electron gun is a source of relatively low velocity electrons directed towards the opposite end of the glass envelope, the latter being enlarged in diameter at 6 to accommodate structure for a target 7. Target 7, in accordance with the present invention, comprises a base or support semiconductor 46 and a sensitive, thin, epitaxially grown semiconductor. layer 45. Only thin layer 45 forms the active target material as hereinafter more fully explained.

The glass envelope 1 houses conducting cylindrical drift space defining walls 8, 9 and 10, maintained at a potential to aid in focusing an electron beam 11 which is produced by the electron gun 4 and focused generally along the axis of said drift space defining walls 8, 9 and 10 by a focusing solenoid 12. Cylindrical drift space wall 8, conveniently formed of copper or copper-plated stainless steel and extending approximately one-half of the length of the focusing solenoid from the electron gun, is joined at the middle of the tube to a double walled drift space section comprising walls 9 and 10. Walls 9 and 10, which accomplish a heat limiting function, extend for approximately the remainder of the focusing solenoid length to enlarged envelope portion 6. Features of the drift space structure are described and claimed in the copending application of R. W. Redington and P. J. Van I-Ieerden, entitled Electron Optics for Infrared Camera Tubes, Ser. No. 56,798, filed Sept. 19, 1960, and assigned to the assignee of the present invention.

Annular decelerating ring 16 in FIG. 1, connected to the appropriate decelerating voltage by means of terminal 17 is joined to flange 14 with sapphire spacing 1 members 18, while additional sapphire members 19 joined to the remaining side of ring electrode 16 are further secured to annular member 20, conveniently formed of copper. Annular member 20 acts to support the target 7 and to conduct heat away from the forward or target end of the tube including especially target 7.

' In the infrared radiation region it is frequently desirable to operate the tube target at temperatures near the temperature of liquid nitrogen or below in order to increase the targets dark current resistivity and therefore sensitivity. To this end, cold finger 21, which may also be formed of copper, is joined to annular member 20 and passes through a seal in the glass envelope 1 to refrigeration means 22. The refrigeration means 22 may be any convenient apparatus for securing to tube target 7, an appropriate operating temperature, e.g., the temperature of nitrogen at its boiling point. Refrigeration means 22 may consist of a Dewar flask containing liquid nitrogen or some other low temperature liquid which may be readily replenished during operation by means not shown. Alternatively, any convenient refrigeration device reaching these temperatures may be utilized, and if desired, a refrigerant can be pumped inside cold finger 21 and around inside member 20.

The tube is directed so that detected radiation falls upon the front of target 7 through window 23, conveniently formed of sapphire for targets which are not sensitive at wavelengths longer than 6 microns, and an optical system (not shown) in front of the tube. The radiation then passes through substantially transparent support layer 46 on the front of target 7 before reaching active layer 45 wherein charge carriers are generated.

Target 7 is positioned within annular member 20 by means of a flanged retaining member 24, shown in greater detail in FIG. 6. The semiconductor target 7 is soldered around its edge to retaining member 24 with indium solder. Retaining member 24 has a cylindrical section 26 with a smaller outside diameter than the inside diameter of member 20 and is secured to member 20 by a radial flange at the end of the cylinder remote from target 7. The radial flange extends over the forward side of annular member 20 so that it may be joined thereto with machine screws 27. These machine screws are tapped into annular member 20 but are electrically insulated from retaining member 24 by sapphire spacers 28 positioned between annular member 20 and retaining member 24, as well as by insulating washers 29, which may be formed of Teflon, disposed between the screw heads and member 24. Holes 30 in the retaining member flange which receive the screws 27, are large enough in diameter so that no electrical connection is made between the flange and the screws.

Conventional deflection coils 31 surround drift space walls 9 and 10, on the inside of focusing solenoid 12 while maze coils 32 and 33 are arranged in that order from the electron gun end of the tube to the deflection coils and also inside focusing solenoid 12. Apertured masks or partitions 34, 35 and 36 are joined to the inside surface of the drift space defining wall 3 at selected intervals therealong, these apertures providing a path for electron beam 11. These coils and masks comprise a maze which, like the aforementioned drift space structure, is useful for a tube operated in the infrared detection region. Features of the maze are also described and claimed in the aforementioned application Ser. No. 56,798.

A connection 39 is made to support base 46 on the front of the target and is coupled to terminal 40, maintained by means not shown at a voltage between approximately zero and volts positive. The voltages at terminals 1,5,17 and 40 are arranged, inter alia, in combined effect for causing electron beam 11 to normally return to electron multiplier 5 in the absence of photoconduction through layer 45 of target 7 in the illustrated embodiment.

The tube together with its focusing solenoid may have a construction similar to but having an overall length approximately twice that of a conventional image orthicon tube, the target being placed at an electron beam focal point twice the usual distance from the electron gun. The deflection coils in the tube may then be the same as in such conventional image orthicon, but with added maze coils of similar construction occupying an approximately equal length between such deflection coils and the electron gun.

In general operation, the camera tube of FIG. 1 functions to receive radiation on layer 45 of target 7 through sapphire window 23 and support layer 46 after passing through an optical system (not shown), this radiation pattern appearing as the variations in the scanned output signal of electron multiplier 5. Electron gun 4 produces a relatively slow stream of electrons deflected into a somewhat spiral pattern and focused at the target 7 back surface by focusing solenoid l2 cooperating with drift space defining walls 8, 9 and 10, the latter being maintained at substantially the same voltage as the electron beam. Maze coils 32 and 33 deflect the electron beam through the apertures in partitions 34, 35 and 36 and thence to the area of the drift space circumscribed by drift space walls 9 and 10 wherein the electron beam is deflected into an appropriate television-type raster by deflection coils 31. The deflection field of these coils is arranged such that the electron beam scans the layer 45 of target 7 depositing or attempting to deposit electrons thereon, while the base 46, functioning as a transparent electrode in the spectral range of interest, is maintained at positive voltage relatively to the electron beam, being on the order ofa plus 50 volts.

7 The epitaxially deposited layer 45 of target 7 is preferably a p-type semiconductor material and therefore a quantum of radiant energy, for example infrared energy, passing through transparent base layer 46 into the epitaxial layer 45, excites a free hole which becomes a current carrier at the point where the radiation strikes. The hole theoretically passes through layer 45 to the electron beam side and neutralizes an electron charge where it passes through. When the picture element is scanned, just enough charge is deposited by the scanning beams to replace the negative charge removed in the preceding frame by the said hole neutralizing an electron, that is by the photoconduction,

If no substantial amount of charge is replaced by the scanning beam at the instant it is directed to the point under consideration, it will substantially fully return by essentially the same path through the tube to the electron multiplier 5. The signal output is then a function of reduction in returned beam electrons caused by illuminated target elements. This electron multiplier output arrangement is desirably included for increasing the sensitivity of the tube although it will be apparent to those skilled in the art that the output signal could be coupled alternatively from connection 39, utilizing an appropriate power supply dropping impedance.

In accordance with an important feature of the present invention, the target 7 comprises the semiconductor base 46 with epitaxially grown layer 45 thereon. The target is illustrated in enlarged form in FIGS. 3 and 7 where the epitaxial layer 45 is discerned as being carried by the base or support semiconductor 46. The epitaxial layer is oriented towards scanning electron beam 11 during tube operation while the base or support semiconductor 46 provides the substantially transparent electrode and window through which the radiation of interest passes in reaching the epitaxial layer. The base semiconductor 46 takes substantially no active part in the photoconductive process itself and therefore the effective semiconductor target thickness is only that of epitaxial layer 45. This layer 45 is typically from It mil to 5 mils in thickness while base 46 is typically 10 to 15 mils in thickness. The thinness of the epitaxial target layer 45 provides a better image resolution because it is easier to keep the image in sharp focus through a thin target layer than a thick target layer. Charge carriers will be generated in a smaller area in a thin target layer for a radiation beam of given dimension. As a simple estimate of size of a resolvable element, we calculate the diameter of a cylinder within which all of the radiation is absorbed. This diameter, d, is approximately equal to L/2fn where L is the effective target thickness;fis thefnumber, and n is the index of reflection.

Also the dynamic range and contrast sensitivity of the target depend on the thickness through the capacitance of the target. The maximum signal current, I, in a vidicon corresponds to the complete discharge of the target between each scan. Thus n mar f where C is the target capacitance, V is the target voltage with respect to the cathode of the electron gun, and T, is the frame time. With a return beam readout and electron multiplier, the noise is the shot noise in the scanning beam. The scanning beam, I is proportional to the maximum signal current,

b 3 mar/" where m is the modulation of the beam at maximum signal. The shot noise is noise mu b V 2e], Af/m where e is the electron charge and Afis the bandwidth. The peak signal-to-noise ratio is then )peak V /26 i The peak signal-to-noise ratio thus improves as the square root of the maximum signal current. Since this ratio determines the contrast sensitivity, it also improves as the square root of the maximum signal current. As far as the tube is concerned, only an increase in the capacitance of the target will have the effect of increasing the maximum signal current, and a reduction in effective target thickness means an increase in target capacitance.

The epitaxial target layer 45 may be deposited upon target base 46 as illustrated in FIG. 2. In FIG. 2, base 46 is supported by receptor 47 located in a heated enclosure comprising a quartz tube 48. The enclosure is located in an oven 49 for heating base 46 to a temperature of from about 830 to 840 C. The base 46 is a germanium semiconductor wafer of from 10 to 20 mils in thickness doped to be p-type or weakly n-type.

In preparing for deposition of an epitaxial layer upon semiconductor base 46, the enclosure 48 is first flushed with dry argon and then hydrogen and the enclosure is heated for a time while dry hydrogen gas passes therethrough. The base 46, previously having had its exposed surface etched and polished as smooth and clean as possible, is now exposed to a vapor etch of HCl carried in hydrogen gas passing through enclosure 48. After thus etching base 46 in place, germanium tetrachloride or another halide of germanium is passed through container 48 in a carrier of hydrogen. At the same time a doping compound adapted to render the deposited germanium of the n-conductivity type is also passed through container 48. Phosphorus trichloride is a convenient doping agent for this purpose. The germanium from the germanium tetrachloride epitaxially deposits upon the heated crystalline base 46 by pyrolytic decomposition. The deposit is said to be epitaxial because the crystalline orientation of the deposit 45 corresponds to the crystalline orientation of the base 46. At the same time, the phosphorus from the phosphorus trichloride provides the initial n-type impurity doping. The growth rate, extent of doping, and the like are controlled by controlling the flow of gases through enclosure 48. It is understood this process is givenby way of example and variations thereof are possible in the attainment of an epitaxially deposited semiconductor layer upon a semiconductor base.

As a result of the foregoing process, an n-type epitaxially grown semiconductor layer is adhered to a p-type or a weakly n-type base. It is now desired to compensate the epitaxial layer so that it is rendered p-type and suitable for responding to impinging radiation. It is desired to approximately just balance the n-type impurity. This balance or compensation would be very difficult to attain during the epitaxial deposition process itself. In accordance with the present invention, the target after epitaxial deposition is metal plated, e.g. with copper. It is feasible to plate the whole target. The target is illustrated in FIG. 4 including copper coating 41. The plated target is heated or roasted for approximately a day or two to allow the metal to diffuse into the semiconductor. The temperature at which this heating is carried out is determined by the initial amount of ntype impurity, e.g., the phosphorus, included in epitaxially deposited layer 45. The latter is conveniently determined by Hall effect measurements. It is desired to add enough copper impurity by a heat diffusion process to balance off the n-type impurity with approximately 95 percent or so as much copper acceptor metal by atomic percentageThe amount of metal added according to the heating process is understood to be a function of the temperature at which the process is carried out. It is therefore determined from solubility curves of a metal in a semiconductor, e.g., copper in germanium. For such a chart, reference may be had to page 86, Vol. 105, Physical Review, Jan. I, 1957, Triple Acceptors in Germanium" by H. H. Woodbury and W. W. Tyler.

After the metal coated target is heated for a day or two, the copper plating is peeled off or removed by hydrofluoric acid and the target is surface etched or undercut to remove a small amount of the semiconductor. Surface etching may be conveniently accomplished with a solution known as CP-4 consisting of hydrofluoric acid and nitric acid.

The resulting target and specifically layer 45 is now compensated and effectively of the p-type and suitable for detecting infrared radiation. Lower energy states of the copper impurity added, act to compensate or trap" electrons from the higher energy state of the ntype donor (phosphorus) material. Copper as a p-type impurity provides another energy level appropriate to infrared detection and also provides sufficient hole mobility and dark current resistivity, particularly at low temperatures. The dark current resistivity of the copper-doped or copper impurity containing germanium increases from about I ohm-cm. at room temperature to greater than 10 ohms cms. at the boiling temperature of liquid nitrogen. Hence refrigeration means 22 is desirable in the FIG. 1. embodiment.

The theory of operation of the invention will be described in connection with energy level diagrams illustrated in FIGS. 7 and 8, showing energy levels for electrons in a germanium semiconductor and specifically including the energy diagram for the support base 46 at the left in these diagrams and the energy levels for the epitaxial layer at the right. Energy is shown as increasing in a vertical direction for electrons and decreasing for holes. In these diagrams the valence band represents energy levels for stable germanium electrons in the non-excited semiconductor. The conduction band on the other hand is a group of normally empty levels to which electrons must be excited (in an intrinsic semiconductor, for example) in order for conduction to take place. The Fermi level is that statistical level below which energy states are most likely to be filled with electrons and above which energy states are likely to be empty. The gap between the conduction band and valence band indicates a usual absence of electrons for such energies in the absence of impurities in the semiconductor. In general, two types of conduction occur in extrinsic semiconductors depending on the impurity added, i.e. conduction of electrons in the conduction band in n-type semiconductors and conduction of holes in the valence band for p-type semiconductors.

Energy states indicated generally at 54 in FIG. 7 illustrate the donor energy states of an n-type impurity, i.e., of the phosphorus, found in the epitaxial layer. In FIG. 7 these energy states contain electrons excitable to energy levels in the conduction band for supporting n-type conduction therein. The support layer on the other hand includes energy states 55 to which electrons from the valence band may be excited for allowing conduction of holes in the valence band.

In accordance with the present invention, first and second acceptor energy states of copper numbered 56 and 57in FIG. 8 are added through the plating and heat treatment hereinbefore described. These energy states, 56 and 57, are added in a quantity sufficient to obtain an atomic percentage of copper approximately equal to the original n-type impurity and approximately balance states 54. The copper states are at about a 0.04 volt and 0.34 volt level, respectively, above the valence band. Electrons formerly occupying energy states 54 in epitaxial layer drop to energy states 56 of the acceptor metal. However the additional energy states 57 of the copper are left vacant, for the most part, and represent a level above the valence band to which electrons may be conveniently raised by the energy of approximately 4 micron infrared radiation. The material doped in this manner is called copper" doped germanium.

Electrons deeper in the valence band raised to energy states 57 will correspond in energy charge to infrared radiation striking a semiconductor at slightly shorter infrared wavelengths. A good response is therefore secured by such a semiconductor target in the range between 1% and 4% microns, a range which is not highly absorbed by the atmosphere and which may be therefore used conveniently for infrared detection, i.e. in viewing objects giving off heat.

It is seen that the addition of the copper acceptor levels to the epitaxial layer have the important effect of rendering the epitaxial layer responsive to infrared radiation for the production of charge carriers flowing therethrough, detectable by the camera tubes electron beam. Nevertheless, the addition of these levels has relatively little effect in the support layer. As can be seen from the energy level diagrams of FIGS. 7 and 8, the addition of levels 56 and 57 has no effect upon the energy band structure of the support layer as far as infrared radiation is concerned. Therefore infrared radiation may pass through a support layer without absorption and the support layer may be used not only for a support for the thin sensitive epitaxial layer, but also as a window for the passage of such radiation and as a transparent electrode for connection to the camera tube circuit.

The target in accordance with the present invention is desirably bombarded upon its scanned surface, i.e. upon the epitaxially grown layer, with sub-molecular particles and specifically noble gas ions after its manufacture, to' prevent a non-imaging state thereof. This treatment is further described and claimed in the copending application of R. W. Redington and P. J. Van Heerden, entitled Photoconductive Camera Tubes and Methods of Manufacture, Ser. No. 56,799, filed Sept. 19, 1960, and assigned to the assignee of the present invention.

While I have shown and described several embodiments of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects; and I therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A camera tube having a photoconductive target having the mechanical strength of a thick target and the electrical characteristics of a thin target and comprising means for providing an electron beam for scanning a target current, said target comprising a radiationtransparent semiconductor base with a thin epitaxiallygrown layer of semiconductor oriented towards said beam for scanning by said beam, wherein only said epitaxially-deposited layer is responsive to radiation detected by said camera tube to alter said beam current, said epitaxially-grown layer being copper"-doped germanium.

2. A camera tube target having the mechanical strength of a thick target and the electrical characteristics of a thin target and comprising a radiationtransparent semiconductor base and a thin epitaxial layer disposed on the side of said base for positioning toward the electron beam of such camera tube, said layer being copper"-doped germanium having first and second acceptor levels, said first level being compensated by n-type impurity in said epitaxial layer.

3. A camera tube target having the mechanical characteristics of a thick target and the electrical characteristics of a thin target and comprising a radiation-transparent semiconductor base and a thin epitaxial layer on said base having a different impurity structure than said base so that said layer is responsive to wavelengths of radiation to which said base is nonresponsive, wherein said epitaxial layer is copper"- doped germanium having first and second acceptor levels, the first of which is substantially balanced by ntype impurity in said epitaxial layer, but wherein said semiconductor base provides insufficient n-type impurity for balancing said first acceptor level.

4. The target according to claim 3 wherein said semiconductor base is p-type,

5. A method of manufacturing a camera tube target having the mechanical resolution and signal-to-noise ratio characteristics ofa thin target which method comprises the steps of epitaxially growing a thin n-type semiconductor layer on a radiation transparent p-type semiconductor target base and adding an impurity to said target rendering said epitaxial layer effectively ptype wherein the impurity added is just sufficient to substantially compensate for the donor impurities in said e itaxial layer. I I

6. he method of claim 5 wherein said epitaxial layer is initially phosphorus doped germanium and said added impurity is copper causing said epitaxial layer to have first and second acceptor levels, one of which substantially balances the donor states of said phosphorus impurity.

7. The camera tube target of claim 3 wherein the target base has a thickness of approximately 10 to 15 mils and the epitaxial layer has a thickness of approximately 0.5 to 5 mils.

8. The method of claim 5 wherein said target base has a thickness of approximately 10 to 15 mils and said epitaxial layer has a thickness of approximately 0.5 to 5 mils. 

1. A camera tube having a photoconductive target having the mechanical strength of a thick target and the electrical characteristics of a thin target and comprising means for providing an electron beam for scanning a target current, said target comprising a radiation-transparent semiconductor base with a thin epitaxially-grown layer of semiconductor oriented towards said beam for scanning by said beam, wherein only said epitaxially-deposited layer is responsive to radiation detected by said camera tube to alter said beam current, said epitaxiallygrown layer being copperII-doped germanium.
 2. A camera tube target having the mechanical strength of a thick target and the electrical characteristics of a thin target and comprising a radiation-transparent semiconductor base and a thin epitaxial layer disposed on the side of said base for positioning toward the electron beam of such camera tube, said layer being copperII-doped germanium having first and second acceptor levels, said first level being compensated by n-type impurity in said epitaxial layer.
 3. A camera tube target having the mechanical characteristics of a thick target and the electrical characteristics of a thin target and comprising a radiation-transparent semiconductor base and a thin epitaxial layer on said base having a different impurity structure than said base so that said layer is responsive to wavelengths of radiation to which said base is non-responsive, wherein said epitaxial layer is copperII-doped germanium having first and second acceptor levels, the first of which is substantially balanced by n-type impurity in said epitaxial layer, but wherein said semiconductor base provides insufficient n-type impurity for balancing said first acceptor level.
 4. The target according to claim 3 wherein said semiconductor base is p-type.
 5. A method of manufacturing a camera tube target having the mechanical resolution and signal-to-noise ratio characteristics of a thin target which method comprises the steps of epitaxially growing a thin n-type semiconductor layer on a radiation transparent p-type semiconductor target base and adding an impurity to said target rendering said epitaxial layer effectively p-type wherein the impurity added is just sufficient to substantially compensate for the donor impurities in said epitaxial layer.
 6. The method of claim 5 wherein said epitaxial layer is initially phosphorus doped germanium and said added impurity is copper causing said epitaxial layer to have first and second acceptor levels, one of which substantially balances the donor states of said phosphorus impurity.
 7. The camera tube target of claim 3 wherein the target base has a thickness of approximately 10 to 15 mils and the epitaxial layer has a thickness of approximately 0.5 to 5 mils.
 8. The method of claim 5 wherein said target base has a thickness of approximately 10 to 15 mils and said epitaxial layer hAs a thickness of approximately 0.5 to 5 mils. 